"Progressively" scanned television receivers have been proposed wherein the horizontal scan rate is multiplied, i.e., doubled, and each line of video is displayed twice thereby providing a displayed image having reduced visibility of line structure. In one form of a progressively scanned receiver, each line of video is stored in one of two memories. As a first of the memories is being written with the incoming video signal at a standard line rate, the second of the memories is read two-times at twice the standard line rate thereby providing two lines of "speed-up" (time-compressed) video within one standard line interval. The second memory output is applied to a display having a doubled horizontal sweep rate synchronized with read-out of the memory thereby doubling the number of displayed lines of video singal. An example of such a progressively scanned receiver, wherein the added lines of video signal are replicas of the original scan lines, is described in U.S. Pat. No. 4,415,931 entitled TELEVISION DISPLAY WITH DOUBLED HORIZONTAL LINES which issued Nov. 15, 1983 to R. A. Dischert.
In another form of progressive scan system, the "extra" lines for display are obtained by interpolation of adjacent vertical lines of the incoming video signal. This may be done either before or after "speed-up" (i.e., time compressing) of the video signal in the memory. An example of a progressively scanned display system in which the additional scan lines are obtained by interpolation from the original scan lines prior to time compression or video "speed-up" is described by K. H. Powers in U.S. Pat. No. 4,400,719 entitled TELEVISION DISPLAY SYSTEM WITH REDUCED LINE SCAN ARTIFACTS which issued Aug. 23, 1983. An alternative of providing interpolation subsequent to speed-up of the video signal is described by Yasushi Fujimura et al. in U.K. Patent Application No. 2,111,343A published June 29, 1983.
It has been recognized (by Powers, for example) that a superior progressive scan image may be obtained in cases where there is no significant scene motion, by using a field memory to delay the incoming video signal by one field. In this way, all 525 lines of an interlaced frame (NTSC assumed) are available for display during each field period thereby avoiding the loss of vertical resolution characteristic of conventional line interpolators. When motion occurs, however, the temporal difference (1/60th. Sec. for NTSC) between the undelayed and field delayed line causes the edges of moving objects to appear serrated.
The serrated edge effect may be corrected as described by Casey in U.S. Pat. No. 4,598,309 filed May 29, 1984 entitled A TELEVISION RECEIVER THAT INCLUDEDS A FRAME STORE USING NON-INTERLACED SCANNING FORMAT WITH MOTION COMPENSATION, commonly asssigned to the assignee of the present invention. In an exemplary embodiment of the Casey apparatus, a frame comb filter is used to delay the incoming composite video signal by one field and to provide a frame comb filtered luminance signal and a motion signal. The chrominance component is removed from the field delayed signal and the resultant luminance component is displayed on alternate lines of a display independently of whether or not motion exists in the scene. The intermediate lines of the display are provided by means of a motion responsive switch that selects the frame comb filtered luminance signal for display during times when little or no motion is present and selects the field-delayed luminance signal (which, additionally, is line-comb filtered) for display otherwise.